N-(phosphonomethyl)glycine (known in the agricultural chemical industry as glyphosate) is described in Franz, U.S. Pat. No. 3,799,758. Glyphosate and its salts are conveniently applied as a post-emergent herbicide in aqueous formulations. It is a highly effective and commercially important broad-spectrum herbicide useful in killing or controlling the growth of a wide variety of plants, including germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants.
Various methods for producing glyphosate are known in the art, including various methods utilizing carbon-supported noble metal-containing catalysts. See, for example, U.S. Pat. No. 6,417,133 to Ebner et al. and Wan et al. International Publication No. WO 2006/031938. Generally, these methods include the liquid phase oxidative cleavage of N-(phosphonomethyl) iminodiacetic acid (i.e., PMIDA) in the presence of a carbon-supported noble metal-containing catalyst. Along with glyphosate product, various by-products may form, such as formaldehyde, formic acid (which is formed by the oxidation of the formaldehyde by-product); aminomethylphosphonic acid (AMPA) and methyl aminomethylphosphonic acid (MAMPA), which are formed by the oxidation of N-(phosphonomethyl)glycine; and iminodiacetic acid (IDA), which is formed by the de-phosphonomethylation of PMIDA. These by-products may reduce glyphosate yield (e.g., AMPA and/or MAMPA) and may introduce toxicity issues (e.g., formaldehyde). Thus, significant by-product formation is preferably avoided.
It is generally known in the art including, for example, as described in Ebner et al. U.S. Pat. No. 6,417,133 and by Wan et al. in International Publication No. WO 2006/031938, that carbon primarily catalyzes the oxidation of PMIDA to glyphosate and the noble metal primarily catalyzes the oxidation of by-product formaldehyde to carbon dioxide, and water. The catalysts of Ebner et al. U.S. Pat. No. 6,417,133 and Wan et al. WO 2006/031938 have proven to be highly advantageous and effective catalysts for the oxidation of PMIDA to glyphosate and the oxidation of by-products formaldehyde and formic acid to carbon dioxide and water without excessive leaching of noble metal from the carbon support. These catalysts are also effective in the operation of a continuous process for the production of glyphosate by oxidation of PMIDA. Even though these catalysts are effective in PMIDA oxidation and are generally resistant to noble metal leaching under PMIDA oxidation conditions, there exist opportunities for improvement.
For example, the distribution and/or size of the pores of the porous substrates utilized in noble metal-containing catalysts may impact catalyst performance and metal utilization. Methods to introduce compounds (i.e., pore blocking compounds) within pores of substrates to modify metal deposition are known in the art. See, for example, U.S. Pat. No. 5,439,859 to Durante et al.
One object of the present invention is development of catalysts effective for the oxidation of PMIDA, formaldehyde, and/or formic acid that more efficiently utilize the costly noble metal, and methods for their preparation. More efficient metal usage may provide catalysts more active than conventional catalysts. Another object of the present invention is development of methods for preparing effective catalysts that require a reduced proportion of costly noble metal as compared to conventional catalysts, while still exhibiting suitable activity.